1. Field of the Invention
The present invention relates to a piezoelectric thin film, a method of manufacturing the same, a piezoelectric thin film manufacturing apparatus and a liquid ejection head.
2. Description of the Related Art
Application examples of piezoelectric devices realized by using piezoelectric thin film include liquid ejection heads for ejecting liquid such as ink. For a piezoelectric thin film to be used for such an application to make its electromechanical transduction feature (of displacement urging pressure) satisfactorily functional, the film preferably has a film thickness not less than 1 μm and not more than 25 μm according to generally accepted theories. Additionally, uniformity is desired to the film for the purpose of accurately controlling displacements.
Known methods of manufacturing piezoelectric thin films include the sputtering method, the metal organic chemical vapor deposition (MDCVD) method and the sol-gel method. With a sol-gel process, firstly a coating solution containing a hydrolizable compound of each of the component metals that operate as starting materials, a partially hydrolyzed compound thereof or a partially poly-condensated compound thereof (piezoelectric thin film precursor) is applied to a substrate and the layer consisting of the coating solution is dried. Subsequently, the dried layer is heated in air and baked at a temperature not lower than the crystallization temperature of the piezoelectric thin film precursor to crystallize the precursor and produce a piezoelectric thin film. Lead zirconate titanate (to be referred to as “PZT hereinafter), a material formed by adding lanthanum to PZT as the third component, which is referred to as “PLZT type” material, or the like is normally employed as the material of piezoelectric thin film. A sol-gel process as described above can form a piezoelectric thin film least costly and most easily.
A metal organic deposition (MOD) process is also known. This process is similar to a sol-gel process. With an MOD process, a coating solution containing a pyrolytic organic metal compound (metal complexes and organic acid salts of metals), for example β-diketone complex of a metal or a metal carboxylate, is applied to a substrate. Then, the applied coating solution is dried in air or in oxygen, for example, to evaporate the solvent in the coating solution and to pyrolize the organic metal compound. Additionally, the piezoelectric thin film precursor is baked at a temperature not lower than the crystallization temperature thereof to crystallize the precursor and produces a piezoelectric thin film.
In this specification document, the sol-gel method, the MOD method and the method realized by combining them are collectively referred to as “the sol-gel method”.
As for the crystalline orientation of a piezoelectric thin film of PZT or the like that is formed by means of the sol-gel method, the direction of polarization moment comes closer to the direction of deformation of the piezoelectric body as the orientation ratio of (100) plane rises relative to the orientation ratio of the other planes ((111) plane, (110) plane, etc.). Therefore, a piezoelectric thin film that represents a high orientation ratio for (100) plane represents deformation to a large extent so that such as piezoelectric thin film can suitably be used as actuator for a liquid ejection head. (See the specification of Japanese Patent No. 3,890,634.)
A piezoelectric thin film formed by using PZT that is produced by way of a sol-gel process is generally arranged on a silicon substrate, a metal substrate, a ceramic substrate or the like (to be referred to simply as “substrate” hereinafter), on which an electrode of platinum or the like has been formed, and then used as piezoelectric device. Substrates as listed above have increasingly been upsized in recent years. In other words, the size of such substrates has been shifted from 4-inch to 6-inch and further to 8-inch. If substrates are upsized, preferably, the orientation ratio of (100) plane is made higher than the orientation ratio of (111) plane and (110) plane and the orientation ratio of (100) plane is practically uniform and invariable throughout the piezoelectric thin film.
However, as substrates are upsized to increase the surface areas of substrates, there arises a tendency that the orientation ratio of (100) plane falls and become remarkably variable from spot to spot in the substrate. Then, as a result, there arises a problem that the electric characteristics of the substrate become variable within the substrate.